Overhead projectors (OHP) are routinely used for the display of text and graphic information contained on a transparency sheet. The transparency sheet is placed on an illuminated stage of the OHP, and the transmitted light collected by suitable optics and projected onto a wall or screen. The transparency images are normally prepared by a thermographic process from originals comprising an infrared-absorbing image, e.g., an ink image or electrostatic toner image containing carbon, on a white background. A translucent substrate bearing a thermosensitive coating is placed in face-to-face contact with the original. Thereafter, illumination with infrared radiation generates heat in the toner image, which in turn records the image on the thermosensitive coating. The thermographic process is usually colour-forming, e.g., a positive, dark-on-light image is recorded. There is a continuing need for single-sheet colour-bleaching materials capable of generating negative images, preferably involving a variety of bright colours, with good resolution and with good pre- and post-imaging stability. Many of the negative-acting systems currently available are of the two-sheet type, in which dye and bleaching agent are coated on separate sheets to preserve shelf-stability. Such materials are less convenient, in both use and manufacture, than single-sheet constructions.
Various single-sheet thermal-dye-bleach constructions are known, notably bleachable antihalation layers for photothermographic elements. Such materials are rarely suitable for OHP transparencies, because the demands made on materials for OHP systems are much more severe. For example, in the unbleached state, the transmission optical density (0.D.) at the wavelength of maximum absorption should be at least 1.0, preferably at least 1.5. Substantially all of this absorption must be discharged by a brief, typically less than 1 second, treatment at moderately elevated temperatures. Longer exposures are unacceptable to the customer, and higher temperatures cause vesiculation of the base (and hence light-scattering and image darkening), unless expensive heat-resistant base is used. Not only must the unbleached colour remain stable under normal storage conditions prior to imaging, but it must also survive prolonged exposure on the OHP subsequent to imaging. This contrasts with the `typical` antihalation layer, where an O.D. of greater than 0.4 is uncommon, heating times of greater than 10 seconds are usual, and bleaching is carried out uniformly rather than image-wise, such that resolution and post-exposure stability are not critical.
U.S. Pat. No. 3,684,552 discloses negative-acting, coloured sheet materials comprising a carrier sheet bearing a heat-sensitive medium comprising an amine-sensitive, coloured dye and a thermal-amine-releaser dissolved in binder. When subjected to imagewise heating, the thermal-amine-releaser decomposes in the heated areas of the material to release an amine compound which reacts with and bleaches the dye, causing a loss or change in colour in those areas. Although the materials are described as being suitable for OHP transparencies, in practice they do not provide an adequate combination of stability and sensitivity.
British Patent Publication No. 2202958 discloses negative-acting, coloured sheet materials comprising a carrier sheet bearing a heat-sensitive medium comprising a coloured dye and a plurality of microcapsules containing a decolourising agent capable of reacting with and bleaching the dye. Imagewise heating causes the microcapsules to rupture or become more permeable in the heated areas of the material, thereby releasing the decolourising agent into the medium to bleach the dye in those areas. Both amines and quaternary amine salts are disclosed as decolourising agents, but only as one of many alternatives, and neither is preferred. The materials are described as being suitable for use as OHP transparencies, but they have the problem of matching the refractive index of the capsules to that of the continuous phase in order to achieve transparency. There are no details of pre- or post-imaging stability.
Japanese Patent Application No. 63-176171 discloses negative-acting OHP sheet materials where the bleaching agent is supplied via a special pen. Such materials cannot be used with conventional transparency imagers.
British Patent Nos. 2150702 and 2173012, and U.S. Pat. Nos. 4,620,204, 4,620,205 and 4,665,410 disclose positive-acting sheet materials capable of forming two different colours when heated at different temperatures. The materials comprise a carrier sheet bearing a heat-sensitive medium containing two dye-precursors, one precursor forming a first coloured dye when heated to a pre-determined temperature and the other a second, different coloured dye when heated to a pre-determined higher temperature. The dye-precursors may be contained in a single layer or two separate layers, with one precursor in each layer. A decolourising agent specific for the first coloured dye is contained in a separate layer adjacent that containing the low-temperature dye-precursor, although where the two dye-precursors are contained in different layers, the decolourising agent may alternatively be contained in the layer containing the high-temperature dye-precursor. An intermediate layer is optionally interposed between the layer containing the decolourising agent and that containing the low-temperature dye-precursor to prevent the migration of the decolourising agent from the former into the latter until the material is heated at the higher temperature.
When imagewise heated at the lower temperature, only the first colour is generated in the heated areas of the material. When heated at the higher temperature, the second colour is also generated in the heated areas of the material, but the decolourising agent migrates across the barrier layer to bleach the first coloured dye.
These materials are primarily intended for use as thermo-sensitive papers, e.g., for facsimile, telex and other information transmission apparatus, and there is no reference to the use of these materials as overhead projector transparencies. The only carrier sheets exemplified in the Examples are "high quality papers".
A wide variety of bleaching agents are disclosed, including both aromatic and aliphatic amines, but the greatest emphasis is placed on polyfunctional amides, generally of high molecular weight, and morpholine and guanidine derivatives. The use of amines is actually discouraged in British Patent No. 2173012. Where the use of amines is taught (e.g., British Patent No. 2150702), solids with a melting point of at least 45.degree. C. are specified. Moreover, while the use of an opaque paper base permits higher imaging temperatures to be tolerated, it effectively precludes application of these materials to OHP systems. The requirement for post-imaging stability is also much less stringent in such materials.
U.S. Pat. No. 3,916,068 discloses heat-sensitive, positive-acting sheet materials comprising a carrier sheet bearing a heat-sensitive medium containing one or two colour-forming "chromagens" and associated colour developers. The heat-sensitive medium either contains two different colour-forming chromagens in a single layer or either the same or a different chromagen in each of two separate layers. An organic amine derivative which is capable of bleaching the chromagen or one of the chromagens is contained in a separate layer adjacent that containing the amine-sensitive chromagen, although, where the chromagen(s) is/are contained in separate layers, the amine may alternatively be contained in one of the chromagen-containing layers.
When the material is heated to a pre-determined temperature, the chromagen(s) and colour developer react to form a coloured image, but when heated to a pre-determined higher temperature, then the amine reacts with the chromagen in the heated regions of the material to bleach the developed colour. In the case where two chromagens are present, one chromagen may generate a first colour at the lower temperature with the second chromagen generating a second, different colour at the higher temperature.
Reference is made to the use of a barrier layer to separate two colour-forming layers, but only to prevent the amine from diffusing into the other chromagen-containing layer, i.e., it is not intended to be selectively permeable when heated.
There is no reference to the use of these materials to form overhead projector transparencies. The only support materials described in the Examples are "ordinary papers".